To evaluate a new petroleum discovery, it is necessary to acquire formation fluid samples for analysis. Often, the only economically viable way to acquire such samples is by open-hole wireline fluid sampling (WFS). However, WFS suffers two sources of error: (1) mud filtrate contamination, and (2) phase separation of formation fluid. The presence of either mud filtrate contamination or phase separation invalidates any ensuing analysis. Some prior art WFS tools provide real-time downhole detection of mud filtrate contamination or phase separation of formation fluid. When mud filtrate contamination or phase separation of formation fluid is detected in a sample, the sample is deemed invalid and is not analyzed.
The Schlumberger Optical Fluid Analyzer (OFA*) and the Schlumberger Live Fluid Analyzer (LFA*) are prior art WFS tools capable of real-time downhole detection of mud filtrate contamination. The OFA* detects water based mud contamination using near-infrared (NIR) spectrum analysis. The LFA* detects oil base mud (OBM) filtrate contamination using gas-oil ratio (GOR) analysis. (OBM has virtually zero GOR). The OFA* and the LFA* also detect one form of phase separation, bubble creation. The LFA* also detects gas phase using three methods. (“*” indicates Mark of Schlumberger). The three methods are: gas-phase detection by change in refractive index, gas-phase detection by temporal variation of methane peak, and gas-phase detection by lack of optical absorption. Neither of these analyzers, nor any other analyzer known to the inventors, provides a method for real-time downhole detection of another form of phase separation, dew precipitation.
The conditions which lead to bubble creation and dew precipitation in formation fluid are illustrated in a pressure/temperature diagram, generally known as a “Phase Diagram”. FIG. 10 (prior art) is a phase diagram showing the conditions under which single phase flow in condensates (3300<GOR<50,000) changes to multi-phase flow under conditions of pressure reduction. It shows bubble creation and dew precipitation. The phase diagram of FIG. 10 shows that when the temperature is below the critical point, and the pressure is reduced, the pressure drop line will intersect the bubble line and some condensates will phase separate as discrete gas bubbles in a continuous liquid phase. The phase diagram also shows that when the temperature is between the critical point and the cricondentherm, and the pressure is reduced, the pressure drop line will intersect the dew precipitation line and some condensates will phase separate as dew (a discrete liquid phase in a continuous liquid phase).
It is also useful to note that petroleum fluids found in subsurface formations can be categorized by their gas/oil ratio (GOR). GOR is expressed in units of standard cubic feet of gas per stock tank barrel of oil, both at 1 atmosphere and 60° F. The categories are: black oils, GOR<2000; volatile oils, 2000<GOR<3300; condensates, 3300<GOR<50,000; wet gas, 50,000<GOR (but finite); dry gas, infinite GOR. The need for detection of dew precipitation in formation fluid exists for a range of petroleum fluid types including volatile oils, condensates, and wet gas.
Wireline fluid sampling (WFS) requires single-phase sampling because if phase separation occurs, then the differential mobility of the phases and the spatial separation of the phases virtually guarantee that the collected sample will not be representative of the formation fluid. Moreover, the process of wireline fluid sampling requires a pressure reduction below formation pressure to move the fluids, and this pressure reduction can cause phase separation. The most common phase separation encountered in WFS is the appearance of a gas phase and a liquid phase. Another common phase separation that can occur with a pressure reduction is asphaltene deposition. For wireline sampling of borehole fluids, it is necessary to recognize two-phase flow when it occurs in order to change flowline conditions to achieve single-phase flow and obtain a representative sample. Generally, pressure is the only adjustable parameter, so the flow type is monitored as a function of pressure. Higher pressure draw-downs are preferred in order to obtain pure formation samples in shorter time by reducing OBM filtrate fractions. However, larger pressure draw-downs are more likely to generate phase separation.
Retrograde condensates are condensates from formations where the temperature is between the fluid critical point (the pressure/temperature point at which distinctions between gaseous phase and liquid phase cease to exist) and the cricondentherm (the highest temperature in which dew is still able to precipitate out of the mixture). Refer to FIG. 10.
For the reasons given above, open-hole wireline sampling of retrograde condensates is unreliable for lack of method and apparatus for timely detection of dew precipitation. Therefore, there exists a need for method and apparatus for downhole detection of dew precipitation.
In a first embodiment illustrated in FIG. 6, the invention uses a measurement of fluorescence intensity, defined by steps 201–210. In a second (preferred) embodiment illustrated in FIG. 7, the invention uses a measurement of fluorescence intensity and a measurement of fluorescence red-shift, defined by steps 301–311. Preferably, the preferred embodiment also uses a measurement of optical absorption. In a third embodiment illustrated in FIG. 8, the invention uses a measurement of fluorescence lifetime, defined by steps 401–410.
Two prior art commercially available tools that allow several samples to be taken from the formation in a single logging run are the Schlumberger Modular Formation Dynamics Tester (MDT*) and the Schlumberger Repeat Formation Tester (RFT*). The MDT* tool includes a fluid analysis module to allow analysis of the fluids sampled by the tool. (“*” indicates Mark of Schlumberger). FIG. 1 of U.S. Pat. No. 3,859,851 shows a schematic diagram of a tool for testing earth formations and analysing the composition of fluids from the formation. The tool of U.S. Pat. No. 3,859,851 is suspended in borehole from the lower end of a logging cable that is connected in a conventional fashion to a surface system incorporating appropriate electronics and processing systems for control of the tool. The tool includes an elongated body that carries a selectively extendible fluid admitting assembly. Such fluid admitting assemblies are shown in U.S. Pat. Nos. 3,780,575; 3,859,851 and 4,860,581. The elongated body also carries selectively extendible anchoring members that are arranged on opposite sides of the body. The fluid admitting assembly is equipped for selectively sealing off or isolating portions of the wall of the borehole such that pressure or fluid communication with the adjacent earth formation is established. A fluid analysis module is also included within the tool body, through which the obtained fluid flows. The fluid can then be expelled through a port back into the borehole, or can be sent to one or more sample chambers for recovery at the surface.
The Schlumberger Modular Formation Dynamics Tester (MDT*) includes a Live Fluids Analyzer (LFA*) that determines the identity of the fluids in the MDT* flow stream and quantifies the oil and water content. In particular, U.S. Pat. No. 4,994,671 (hereby incorporated herein by reference) describes a borehole apparatus which includes a testing chamber, means for directing a sample of fluid into the chamber, a light source preferably emitting near infrared rays and visible light, a spectral detector, data base means, and processing means. Fluids drawn from the formation into the testing chamber are analysed by directing the light at the fluids, detecting the spectrum of the transmitted and/or backscattered light, and processing the information accordingly (preferably based on information in the data base relating to different spectra), in order to quantify the amount of water and oil in the fluid.